“For this, you would need warm summers, with not too much precipitation,” Nils Christian Stenseth, an author of the study, told the BBC. “… And we have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather.”
Instead, the fearsome “Black Death,” as the epidemic was known, seemed curiously tied to the climate in Asia. Analysis of 15 tree-ring records, which document yearly weather conditions, shows that Europe always experienced plague outbreaks after central Asia had a wet spring followed by a warm summer — terrible conditions for black rats, but ideal for Asia’s gerbil population. Those sneaky rodents and their bacteria-ridden fleas then hitched a ride to Europe via the Silk Road, arriving on the continent a few years later to wreak epidemiological havoc.
The findings absolve Europe’s black rats of responsibility for the deaths of more than 100 million people in the “second plague pandemic,” which began with the Black Death in the mid-14th century and recurred until the 1800s. They also explain why the disease popped up intermittently century after century, rather than lingering on the continent as long as rats were around to carry it.
This isn’t the first time scientists have challenged a popular understanding of the disease. Last year, researchers examining plague DNA found in 25 14th-century skeletons said they found evidence that the disease was airborne rather than distributed via flea bites.
So Stenseth says his team will fact-check their findings by analyzing DNA from a variety of ancient European skeletons. If the samples show significant genetic variation across time, that would indicate successive outbreaks were caused by newly arrived waves of the disease rather than a resurgence from the continent’s rat reservoir.
“If we’re right, we’ll have to rewrite that part of history,” Stenseth said.
And hundreds of elementary school classrooms will have to rethink their class pet.